I. Introduction
In RECENT years, there has been a growing interest in the condition assessment of transformer insulation. Primarily this is due to the increasingly aged population of transformers in utilities around the world. A large proportion of existing power transformers within electric utilities are approaching the end of their design life. Many, perhaps most, seem to be operating satisfactorily. However, insulation degradation continues to be a major concern for these aged transformers. Insulation materials degrade at higher temperatures in the presence of oxygen and moisture. The degradation from thermal stress affects electrical, chemical, and mechanical properties [9].